302 Dr. T. Williams on the Mechanism of Aquatic 



No setse or bristles of any description are added to these 

 simple organs. They are enveloped in an exquisitely-attenuated 

 tessellated epithelium. It preserves its tenuity without a sacrifice 

 of stiffness. These organs are very readily converted into vesicles 

 by compressing the body of the animal. The force of the fluids 

 separates the pai'allel laminae, and converts a plane into a sac. 

 They are moved by muscles at the base ; they are attached to 

 the second and third segments of the thorax. The action of 

 the water upon them is auxilialed by flabellse. They exhibit the 

 apparatus of breathing under the characters of the greatest 

 simplicity. 



The common Talitrus will serve to illustrate the anatomy of the 

 branchial parts in the Amphipodan family. The thoracic limbs 

 are commonly said to be transformed into branchiae at their 

 bases. The depending edges of the dorsal plates (the epimeral 

 pieces of the tergal arc) are however much more suitably or- 

 ganized than the proximal articulations of the legs (PI. XVIII. 

 fig. 1, c). They are penetrated by a very dense system of canals 

 {b, c). The epidermis is reduced to an extremely thin and 

 transparent lamina. The component hexagonal cells may be 

 readily observed (fig. 2). The outer or epidermal lamina is 

 united to the opposite parallel lamina by dots of parenchyma {a). 

 The blood streams in the intermediate passages [b) . These parts 

 therefore correspond in ultimate structure in the most exact 

 manner with leaves of the branchiae of the Crab. The bases of 

 the legs are filled with muscle fasciculi. In Talitrus, as in 

 all Crustacea, the blood-cun-ents are large and few in number. 

 No setae or bristles of any sort belong to these latei*al branchial 

 plates. The respiratory current is maintained by the action, 

 which is ceaseless, of the three pairs of abdominal prolegs. In 

 several orders of this family the flabellse of the abdominal ap- 

 pendages ai*e converted into branchiae (PI. XVII. figs. 4 & 5). In 

 minute structure they coincide with the lateral respiratory 

 plates of Talitrus. From these parts, when thus specialized for 

 breathing, bristles are absent. The associated palp excites the 

 aerating current. 



In the family of Stomapoda, the species of which are rarely 

 found in the British seas, each segment of the abdomen is fur- 

 nished with a pair of broad natatoiy feet, the basilar joint being 

 quadrilateral (PI. XVII. fig. 3, a), each bearing two lamellar 

 branches {d), the exterior of which gives attachment on its 

 posterior face, and close to the peduncle, to a tufted branchia (Z>). 

 The minute structure of the branchial tubuli conforms in every 

 respect with those of the Lobster, afterwards to be described. 

 Each tubule is traversed in its centre by an afferent column of 

 blood, which breaks out into a network along the circumference 



